1. Field of the Invention
The present invention relates to the field of acoustic sensors, and more particularly to a distributed sound gathering device with an interferemetric fiber optic sound transducer.
2. Description of the Related Art
Point acoustic sensors are generally referred to as microphones, hydrophones, geophones or the like depending on the medium in which they are designed to operate. Such devices generally are placed at a single point and respond to acoustic waves which travel to them through the medium, such as air, from a remote source. The acoustic wave which impinges on the sensor is much weaker at the sensor than at the source for two primary reasons. First is the dispersion of energy from the source which causes the power density of the acoustic field to fall off approximately as 1/R.sup.2 where R is the distance from the source. Second, acoustic energy is dissipated by the lossy characteristics of the medium as the acoustic waves travel through the medium. This loss is often referred to as attenuation. The sum of these two primary effects and others is that the intensity of the acoustic energy is much reduced at points remote from the source, and the degree of reduction is dependent on the physical properties of the medium.
It is well known that water is an excellent acoustic conductor, air is a fair conductor, and most granular soils are very poor conductors. As a result, microphones in air are normally used in close proximity to the source such as a speaker, whereas hydrophones may be used to collect sounds in water from sources at great distances. On the other hand, microphones, or more aptly geephones, are not much used in soils because of the high attenuation and dispersion of sound in soils. However there are numerous applications where effective acoustic sensors in soils would be very desirable. Notable examples are systems for detection and identification of vehicular traffic passing over the ground, or systems which can detect intrusions into protected areas such as public utility facilities or government reservations. There are often physical or operational reasons why the sensors must be located at considerable distances from the sources of interest. This necessarily means that the signals at the sensor are weak, and there is much intervening space in which extraneous noise can intrude. While the low signal levels might be overcome by use of very high amplification, this is often ineffective because of the high noise mixed with the signal which is equally amplified. Moreover, when high amplification is used in conjunction with conventional electromagnetic transducers, there is the additional severe problem of inductive pick-up of electromagnetic noise from sources such as power lines.
An effective approach to overcome these problems is to provide a widely distributed sensor in the soil which can respond to acoustic inputs nearer their source over a large area and which is not susceptible to electromagnetic interference. One such approach is the Distributed Fiber Optic Sensing System disclosed in copending U.S. patent application Ser. No. 07/903,358 filed on Jun. 24, 1992 by Brian B. Crawford et al and assigned to the present assignee. This disclosure is incorporated herein by reference. The distributed sensor of that application is an extensive loop of optical fiber cable which is operated as the sensitive element of a fiber Sagnac interferometer. This loop may be up to several tens of kilometers in length and may completely surround a facility to be monitored. It is convenient to either bury the sensing fiber or incorporate it into an existing structure such as a fence. In this case the fiber is a very low loss optical waveguide which is sensitive to acoustic or mechanical disturbances over its entire length, and conveys the signal back to the signal analysis center with very little attenuation or additional noise.
The applications and advantages of such a distributed Sagnac fiber sensing system are set forth in U.S. patent application Ser. No. 07/903,358. However, some improvements are desirable in certain applications. In particular, the burial and protection of the extensive fiber loop may be too expensive for some situations. Also, there are occasions where higher sensitivity in a more localized area may be desirable, for example under a specific road. Another desired feature is the possibility of economical placement of multi-element arrays to provide location and ranging information with appropriate signal processing means. What is desired and has not yet been provided by the prior art are improvements to distributed fiber sensing systems which provide low cost distributed sound collecting means, higher sensitivity for low level disturbances, and practical means for multi-sensor directional arrays.